The diphenol 2,2 bis-(4-hydroxyphenyl)propane, otherwise known as bisphenol-A, is a well known phenol in the commercial world. It is used in the preparation of many polymers including polyarylates, copolyester-carbonates, polycarbonates, epoxies and other polymeric materials. In order to make polycarbonates, a high quality bisphenol-A is required. Therefore, substantial attention has been directed to the methods of preparing and isolating relatively pure bisphenol-A. Researchers in the past have taken advantage of the fact that bisphenol-A and phenol, one of the two starting materials in preparing bisphenol-A, form a 1:1 molar adduct which lends itself to purification. Processes which start with the isolation and purification of bisphenol-A through the treatment of an adduct are disclosed in U.S. Pat. No. 4,209,646, Gac, et al, issued to Rhone-Poulenc Industries and U.S. Pat. No. 4,212,997, Adams, et al, issued to General Electric Company. In Gac, impure bisphenol-A, phenol and less than about 15% by weight of water is brought to a temperature wherein the mixture is entirely liquid. It is then cooled to a temperature less than about 60.degree. C. while applying reduced pressure to the mixture to form the adduct of bisphenol-A and phenol which crystallizes. The phenol is then removed from the bisphenol-A so as to obtain the bisphenol-A apart from phenol. The temperature which is generally necessary to maintain the mixture in this liquid state prior to the adiabatic cooling step is from about 70.degree. to 100.degree. C. In Adams, et al an adduct between the bisphenol-A and phenol is also the starting point in the process. The temperature of the mixture is lowered from the point at which bisphenol-A is soluble to a temperature from which the adduct crystallizes. The adduct is recovered and mixed with an excess of water at a temperature to completely melt the adduct. The water present is an amount sufficient to retain the phenol portion of the adduct upon separation of bisphenol-A from the phenol, and then lowering the temperature of the water and melted adduct to cause the separation of the bisphenol-A portion of the adduct from the phenol.
Additionally, U.S. Pat. No. 3,326,986, Dugan, et al, issued to Dow Chemical Company shows the crystallization of bisphenol-A from its isomers and excess phenol through the addition of water and heating to a temperature sufficient to melt the crude bisphenol-A and other impurities, followed by cooling to affect the crystallization of the bisphenol-A. Sufficient phenol is present to form a substantial amount of adduct.
It has now been found that substantial quantities of adduct of bisphenol-A and phenol are not necessary to prepare crystalline bisphenol-A in pure form. Rather, an impure form of bisphenol-A with essentially no phenol present can be the starting point for crystallization and purification steps. This allows for the preparation of very pure bisphenol-A in high yield from the crude bisphenol-A prepared by art known methods. The process also has the advantages of being done in a continuous phase if desired.